For forming patterns in structures in the micrometer and sub-micrometer range for electronic and micro technology applications usually methods relying on electron beam, X-ray, ion beam and optical lithography are employed. With these lithographic methods a resolution of 1 micrometer or better can be achieved. The methods are based on a step-and-repeat process and are thus not compatible with continuous in-line manufacturing. At the moment, a throughput of 10−3 m2/s, which corresponds to 50,000 300 millimeter wavers per month, can be achieved with these lithographic methods, the required equipment being rather expensive.
Traditional high-throughput print technologies such as offset, flexography, and gravure—a type of intaglio print process—have been employed for patterning and forming electronic circuits. Offset printed transistors have been reported in Y. Mikami, IEEE Transactions on Electron Devices 41, 306 (1994) and by D. Zielke et al, Applied Physics Letters 87, 123508 (2005), for example. Transistors structured by flexographic printing have been reported by T. Mäkelä et al., Synthetic Metals 153, 285 (2005). An intaglio print process is disclosed in WO 2004/021751 A1. Processes employing these print technologies offer a throughput of several m2/s but their resolution is currently limited to 20 micrometers or more, which is usually not sufficient for electronic applications.
Another known method for forming patterns with nanometer resolution on a substrate is the so-called nanoimprint lithography (NIL) (e.g. U.S. Pat. No. 5,772,905 A). Nanoimprint lithography is a process wherein a mould is is pressed into an imprint polymer. Nanoimprint lithography allows in principle for parallel, arbitrarily complex patterning with less than 10 nanometer resolution. The imprinted polymer layer is usually used as an etch resist in a subsequent process step and the pattern is transferred into an underlying layer by etching. Typically, the mould has to be reconditioned or even replaced after a fairly limited number of imprint processes. Furthermore, during the imprint process, material is usually flowing from the raised regions of the pattern to the mould's recesses. Depending on the pattern to be imprinted as well as the imprint material, the process time of nanoimprint lithography might be quite large.
Solid-state embossing is another known method for forming a pattern on a substrate. It is a non-lithographic patterning technique that is often used for manufacturing diffraction gratings, compact discs, and security features such as holograms. Nanoscale patterns may be formed. The process consists of forcing a microcutting tool that is normally made from silicon and comprises an array of protruding wedges into a multilayer structure. In WO 02/29912 A1 solid-state embossing is employed for manufacturing electronic circuits. It can be used to “cut” transistor channels into a single conducting layer supported by a PET (polyethylene terephthalate) substrate. Channel lengths of down to 12 micrometers can be achieved. The method can also be employed for the manufacture of vertical-channel transistors. A vertical-channel transistor is defined such that the source and the drain electrodes are positioned on top of each other and the channel length is defined by the thickness of an insulating layer. As the thickness of the insulating layer can be well controlled even on a sub-micrometer length-scale, vertical architectures are highly relevant for short channel length transistors. In N. Stutzmann et al., Science 299, 1881 (2003) solid-state embossing is used for controlled microcutting of vertical sidewalls into a conductor-insulator-conductor trilayer structure. Vertical channel transistors with channel lengths of down to 0.7 micrometer can be generated. Solid-state embossing is usually quite time consuming and the employed microcutting tool has to be robust, especially for a large-scale production facility.
Another known method for forming and patterning high-resolution structures is so-called microcontact printing. Microcontact printing is based on the selective transfer of an organothiol or a silane to a substrate using an elastomeric stamp. The areas of the substrate that have been contacted by the raised surfaces of the elastomeric stamp are covered with a self-assembled monolayer of the organothiol or the silane and can be used as etch resist in a subsequent processing step. The resolution of the method is mainly determined by the resolution of the elastomeric stamp. The elastomeric stamps are usually fabricated from poly (dimethyl) siloxane (PDMS), polyimide, or phenolformaldehyde polymer. Microcontact printing is usually followed by either etching, electroless plating (e.g. U. Zschieschang et al., Advanced Materials 15, 1147 (2003)), or area-selective electropolymerisation (e.g. C. B. Groman et al., Chemical Materials 7, 526 (1995)) for forming electronic circuits. M. Leufgen et al., Applied Physics Letters 84, 1582 (2004) describes microcontact printing followed by etching for forming transistors with down to 100 nanometer channel length. WO 03/099463 A1 discloses a wave printing technique for automated patterning with microcontact printing. J. Schellekens et al., Materials Research Society Symposium Proceedings EXS-2. M2.9.1 (2004) describes transistors with down to 1 micrometer channel length, which have been microcontact wave printed onto 150 millimeter glass wavers. E. Kim et al., Applied Physics Letters 80, 4051 (2002) describes a method based on microcontact printing for direct patterning of a metal layer. A metal layer coated on a pre-patterned poly dimethyl siloxane stamp is transferred to a substrate by cold welding. When removing the stamp, the metal on the raised regions of the stamp adheres to the substrate and a metal pattern is formed. The inverse process where a stamp is used to lift-off a pattern from a homogenous metal layer is described in T. Wang et al., Advanced Materials 15, 1009 (2003). EP 0 953 420 A2 discloses so-called micromoulding in capillaries (MIMIC) for patterning polyaniline and carbon structures. A mould is put in conformal contact with a substrate, thereby forming microchannels. Capillary forces pull an ink solution into the channels. Solidification by evaporation of a solvent or thermal conversion followed by removal of the mould completes the fabrication of the microstructures. For micromoulding the electrode layout must fulfil tight requirements for the ink to actually flow by capillary forces. Because of the ink flow and the drying step micromoulding is a comparably slow process.
U.S. Pat. No. 6,946,332 B2 extends the microcontact printing concept to nanoscale dimensions in an additive process referred to as nanotransfer printing. In nanotransfer printing metal structures are directly transferred onto a substrate surface. Patterning of features with sizes approaching 100 nanometers and an edge resolution of less than 15 nanometers is achievable. WO 2004/004025 A2 discloses direct patterning of water-based conductive inks via a self-assembly process based on microcontact printing. A substrate is pre-patterned into hydrophobic and hydrophilic regions by microcontact printing of a hydrophobic self-assembled monolayer. When applying water-based ink, it spontaneously dewets from the hydrophobic regions, thereby forming material free channels.
Summarizingly, microcontact printing-based methods offer a resolution of down to below 100 nanometers. Microcontact wave printing allows for patterning of micron-sized features with an overlay accuracy of more than 2 micrometers over a 150 millimeter waver (M. M. J. Decré et al., Materials Research Society Symposium Proceedings EXS-2. M4.9.1. (2004)). The printing contact time for the 150 millimeter waver is reported to be 15 seconds which corresponds to a throughput of 10−3 m2/s, not taking into account the stamp soaking and drying time.
Ink-jet printing is another known patterning method. Ink-jet printing is intrinsically a non-contact, additive technique. Sacrificial resist or lift-off layers are not required. Material is only deposited where it is actually needed. With ink-jet printing droplet volumes of down to 10 picoliters can be achieved which corresponds to a diameter of about 20 micrometers. Due to statistical variations of the flight direction of the droplets and the spreading on the substrate, the smallest features and gaps currently attainable with ink-jet printing are about 50 and 20 micrometers, respectively. Combining ink-jet printing with other patterning techniques that offer a resolution of below 10 micrometers, as taught for example in WO 01/46987 A2, may improve the method. Here the substrate is pre-patterned into hydrophobic and hydrophilic regions prior to the ink-jet deposition. The droplets are repelled by the hydrophobic regions and, hence, confined by their edges. The pre-patterning into hydrophilic and hydrophobic regions can be achieved by using self-assembled monolayers of a perfluorinated silane (WO 2005/038881 A2) or by a direct-write process based on laser patterning (WO 02/095805 A2). WO 03/056641 A1 discloses furthermore a lithography-free self-aligned ink-jet printing process, wherein a first conductive pattern is ink-jet printed onto a substrate, the surface of the first conductive pattern is selectively modified to be of low surface energy by using CF4 plasma or by adding a suitable surfactant to the ink while not modifying the surface of the substrate, and a second conductive pattern that partially overlaps the first conductive pattern is ink-jet printed, not requiring precise relative alignment. The droplets of the second conductive pattern are repelled by and flow off the low energy surface of the first pattern and dry with their contact line in close proximity to the edges of the first pattern, thereby forming a small self-aligned gap. Transistors with less than 100 nanometer channels may be obtained.
U.S. Pat. No. 3,134,516 discloses the use of ink-jet printed hot-melt wax as etch resist that solidifies on contact with the substrate. Spreading of the printed droplets can thus be prevented. Ink-jet printed transistors with 30 to 50 micrometer channel length can be realized.
Ink-jet printing is intrinsically a serial process. Its throughput is nowadays limited to about 0.01 m2/s.
Thermal imaging can also be used for forming patterns on a substrate, see for example U.S. Pat. No. 5,523,192 A, WO 02/070271 A2, or WO 2004/087434 A1. Thermal imaging is a dry, solvent-free digital printing process in which functional materials are transfer printed, layer by layer, from donor sheets onto a substrate using localized laser-induced heating. With a method based on thermal imaging transistors with down to 5 micrometer channel length can be produced. A resolution of less than 10 micrometers has been achieved over areas of more than 3 m2. The materials, however, have to withstand high temperatures. Thermal imaging is intrinsically a serial process and its throughput is currently limited to about 0.002 m2/s.